The melt extrusion of high molecular weight polymers, for example, hydrocarbon polymers and polyamides, into shaped structures such as tubing, pipe, wirecoating or film is accomplished by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form and is then subsequently cooled and solidified into a product having the general shape of the die.
In order to achieve low production costs, it is desirable to extrude the polymer at rapid rates. Higher extrusion rates may be readily obtained by increasing the rate of revolution of the extruder screw. However, this technique is subject to limitations imposed by the viscoelastic properties of the polymer substrate. Thus, at very high extrusion rates an unacceptable amount of thermal decomposition of the polymer can result. Further, extrudates having a rough surface are often obtained which can lead to formation of an undesirable pattern on the surface of the extrudate. Extrusion at elevated temperatures obviates this problem but adds to processing costs. Also, cooling of the extrudate becomes problematic. In addition, if polyolefins are extruded at temperatures near their decomposition points, polymer degradation occurs.
It is desirable, therefore, to find highly efficient means of increasing the extrusion rate without raising the melt temperature, while producing articles having smooth surfaces. Changes in extruder and die configuration can improve polymer melt flow, but these modifications are not always practical or economically feasible. Another approach involves the addition of conventional wax-type process aids which reduce bulk viscosity and in some cases improve processing properties. However, the efficiency is marginal and the high levels of additive required often adversely affect other properties.
In Blatz, U.S. Pat. No. 3,125,547, it is disclosed that the use of 0.01-2.0 wt. % of a fluorocarbon polymer that is in a fluid state at process temperature, such as a fluoroelastomer, will reduce die pressure in extrusions of high and low density polyethylenes and other polyolefins. Further, use of this additive allows significant increase in extrusion rates without melt fracture.
Kamiya and Inui, in Japanese Patent Application Publication Kokoku 45-30574 (1970, examined), cite the use of crystalline fluorocarbon polymers at temperatures below their melting points to eliminate die build-up, but they disclose nothing regarding other extrusion improvements.
Nishida, et al., in Japanese Patent Application Publication Kokai 62-64847, disclose injection molding compositions comprising a mixture of a) an ethylene/alpha olefin copolymer having a melt flow rate (MFR) of 0.2-200 g/10 minutes and a density of 0.850-0.945 g/cm.sup.3, with b) 0.001-1% by weight of a fluorinated hydrocarbon polymer having a fluorine to carbon ratio of at least 1:2.
Chu, in U.S. Pat. No. 4,740,341, discloses blends having improved extrudability comprising linear polymers of ethylene having incorporated therein small amounts of fluorocarbon polymers and polysiloxanes. The fluorocarbon polymers have fluorine to carbon ratios of at least 1:2 and are fluid at 120.degree.-300.degree. C.
Larsen, in U.S. Pat. No. 3,334,157, discloses polyethylene which has been modified to improve its optical properties by incorporation of 0.015 to greater than 1.7% by wt., based on the mixture, of finely divided polytetrafluoroethylene.
More recently, improved fluoropolymer process aid compositions have been disclosed in for example, U.S. Pat. Nos. 5,464,904; 5,132,368; and 5,587,429.
In order to maximize processibility improvements, it is desirable that the fluoropolymer process aid compositions be well dispersed in the non-fluorinated polymer which is to be extruded. Generally, the smaller the particle size of the fluoropolymer, the better the dispersion. However, it is quite difficult to prepare process aid masterbatches which contain small particle size fluoropolymers using conventional mixing techniques.
It is an object of this invention to provide non-fluorinated resin compositions having incorporated therein well-dispersed fluoropolymer process aids, thereby resulting in extrudable melt-processible compositions having substantially improved processability characteristics. That is, it is an object of the invention to provide melt-processible compositions which can be extruded at rapid rates, low die pressures, and low melt temperatures to provide shaped articles having excellent surface quality.